1. Field of the Invention
The present invention relates to a polymer insulator having a core member, an insulation overcoat member arranged on an outer surface of the core member, and a securing metal fitting fixed to an end portion of the core member in such a manner that the end portion is contacted with the insulation overcoat member.
2. Description of Related Art
FIG. 2 is a cross sectional view showing one embodiment of a known polymer insulator. In the embodiment shown in FIG. 2, a polymer insulator 1 comprises an FRP rod 2 as a core member, an insulation overcoat member 3 made of rubber such as silicone rubber which is arranged on an outer surface of the FRP rod 2, and securing metal fittings 4 which are secured and fixed to both ends of the FRP rod 2. The insulation overcoat member 3 comprises a sheath portion 5 and a plurality of sheds 6. In order to produce the polymer insulator mentioned above, the insulation overcoat member 3 is molded on the FRP rod 2, and then the securing metal fittings 4 are secured and fixed to both ends of the FRP rod 2. In this case, a seal portion 7 (FIG. 3) made of a sealing agent such as rubber of silicone system is arranged at a boundary between the insulation overcoat member 3 and the securing metal fittings 4, which is exposed to an external atmosphere, so as to prevent an inclusion of water or the like through the boundary.
During a normal field test, the known polymer insulator having the construction mentioned above shows no problem on tracking-erosion properties of the insulation overcoat member 3 and the seal portion 7. However, if the known polymer insulator is used under a severe fouling condition, or, if the known polymer insulator is subjected to an acceleration damage test, there is a case such that an erosion occurs at the seal portion 7. Severe fouling conditions as know in the art include coastal areas (fog, salt), deserts (dust), and so forth. Therefore, in order to improve a reliability of the polymer insulator, it is necessary to improve tracking-erosion resistant properties much more.
Moreover, the seal portion 7 is arranged at both ends of the polymer insulator 1, to which a high electric field is liable to be applied. Therefore, as shown in FIG. 3, corona or dry-band-arc is liable to be generated at the seal portion 7, especially if the polymer insulator 1 is used under the severe fouling condition. Owing to this, in the seal portion 7, it is necessary to have excellent tracking-erosion resistant properties in addition to sealing properties which are same as those of the known polymer insulator.